The present invention refers to an improved magnetic holding device such as a magnetic chuck or work-holder used on lifting apparatus and machine tools, and particularly relates to a magnetic holding device provided with pole extension members to adeguately support and magnetically retain iron pieces.
In the field of magnetic work-holding apparatus, for example for holding work-pieces on machine tools, there is the problem of holding the work-piece without causing any distortion while the piece is being machined or when the work-piece is magnetically gripped on the activated device, due to magnetic attraction forces on the piece by the holding surface. A magnetic work-holding apparatus is described for example in U.S. Pat. No. 4,356,467 and U.S. Pat. No. 4,507,635 of the same inventors.
This problem exists in the majority of cases and is due to the elastic deformation caused by magnetic forces on the work-pieces which tend to reassume their initial shape when they are released, defeating all working precision.
There is at present no specific system for solving this problem, except for placing suitable wedges or shimming pieces between the piece or the pieces to be held and the holding surface of the magnetic device, so that any distortion of the pieces is limited, by preventing them from adapting to the holding surface.
The efficacy of this type of system is nevertheless extremely limited in that mechanical shimming lacks precision, involves additional air gaps that worsen circulation conditions of the useful magnetic flux and it cannot be practically applied to pieces with large surfaces both because of difficulty in ascertaining the exact points where the shims should be placed and because of difficulty in inserting the shims.
Another known system provides for lateral clamping of the single pieces to avoid any compression towards the holding surface; however, this system prevents high working forces from being developed in any case so as to cause distortion and makes it necessary to work on the piece repeatedly in a light manner, until a face that will then serve as a final reference face for later machining is achieved.
In the field of magnetic lifting apparatus it is also know the use of pole-extension pieces that are mobile only in a longitudinal direction to improve magnetic flux circulation conditions, without actually contributing to holding the piece to be lifted, since they do not afford sufficient force to hold back the piece in the sliding direction, unless they abut onto stop surfaces; in this way no adjustment of the pole piece position is possible. Practical results obtained with similar apparatus are scant and cannot be adopted on machine tools.
From U.S. Pat. No. 3,089,986 it is also known a multipole magnetic jig for use with magnetic chucks in which the jig comprises a lower portion and an upper mobile portion having a plurality of pole-extension members to adapt to the top plate of a magnetic work-holding device. The upper and lower portion of the jig are abutting on slanted or inclined surfaces allowing the upper portion to slide on the lower portion to adjust the high of the jig; a slug is forced into a hole of the lower portion urging against a side of a guiding ridge of the upper portion to prevent movement in the adjusted condition. Therefore U.S. Pat. No. 3,089,986 does not suggest or describe a magnetic apparatus having pole-extension members automatically adapting to the shape and local deformations of a work-piece to be hold.
An object of the present invention is to provide a work-holding apparatus having pole-extension members that automatically adapt to the surface of the pieces to be held, allowing high circulation of useful flux to minimize dispersions, allowing at the same time a firm hold of the pieces.
A further object of present invention is to provide a single-pole extension members of the type referred to, particularly suitable for application on pole pieces of a magnetic holding surface of a work-holding apparatus for machine tools and the like, that make it possible to operate with high working forces allowing considerable amounts of material to be removed right from the beginning of each machining and the piece to be firmly held and supported so that it is not subjected to any distortion. In this way the precision of matching is highly improved and made comparable to the machine tool's tolerances. Moreover optimal flux circulation conditions can be obtained by eliminating air gaps between the pole extension members and the work-piece, thus increasing the force by which the work-pieces are held and the working ability of the machine tool, it being possible to remove more material from the work-piece at each cut of the tool than is possible with known systems.
Yet a further object of the present invention is to provide a magnetic holding device provided with pole extension members appropriately arranged on the aforesaid adhering surface of a work-piece to ensure that pieces of any size are held firmly and securely, preventing any displacement by the forces caused by the working tool.